In other words, think of Copyright. A Copyright holder can apply any license they like, and change the licenses for new versions whenever they like. Public Domain is explicit forfeiting the Copyright, which means authors cannot enforce any license (and anyone can just take their work and declare it it's theirs, apply licenses etc).

PS: AFAIK, however, Authorship rights are different from Copyright, and cannot be given/passed as Copyrights, at least in US.